1. Field of the Invention
The present invention relates to a polarizer within an inspection system and in particular to a composite polarizer having a plurality of plate polarizers, each plate polarizer having a predetermined pitch angle and roll angle to provide a desired polarization.
2. Related Art
In general, using a polarizer in the imaging or collection path of a wafer inspection tool can take advantage of the distinctly different polarized scattered light from a wafer surface and from defects to significantly improve detection sensitivity. In a conventional wafer inspection tool, a linear polarizer with a unified polarization across the entire imaging aperture is typically used in the collection path. Note that a linear polarizer can be a reflective polarizer in which an incoming beam is split into a transmitted beam and a reflected beam with perpendicular polarization states, or an absorptive polarizer in which one of the two perpendicular polarizations is absorbed and the other one is transmitted. Reflective polarizers generally have much higher efficiency but need a larger free space because of the large incident angle. Absorptive polarizers are more compact since it can operates at a normal incident angle, but are much less efficient than reflective polarizers because of absorption loss.
Another type of polarizer used in wafer inspection is a linear polarizing cube with a single unified polarization. Notably, this linear polarizing cube has an adjustable polarizer angle, but the polarization is at a constant angle over the entire imaging aperture. Therefore, a linear polarizing cube cannot achieve a maximum signal to noise ratio (SNR). In one embodiment to achieve improved SNR, a linear polarizing cube can be cut diagonally from opposite corners and then reconnected to form the cube. By having multiple reconnected polarizing cubes available in a wafer inspection system, it is possible to have multiple reconnected polarizing cubes of which polarizations are at different pre-determined angles, wherein switching between the reconnected cubes can provide different polarizations. Such a system is complex and bulky, and polarization angles can only be adjusted in a set of discrete steps but not continuously, which makes it difficult to optimize for maximum SNR.
A so-called “pizza pie” polarizer has been used for wafer inspection, wherein this polarizer is formed with multiple absorptive linear polarizers (each shaped like a pizza pie piece) to provide a circularly symmetric polarization arrangement. These linear polarizers have either polarization aligned in a radial direction or in a tangential direction. One of the disadvantages of the conventional “pizza pie” polarizers is the low efficiency due to the use of absorptive polarizers.
Unfortunately, a need still exists for a polarizer that has adjustable polarization angles while remaining mirror symmetric with a plane, e.g. the incident plane of a wafer inspection system with oblique illumination.